Abstract
CRISPR-associated proteins (Cas1 and Cas2) integrate foreign DNA at the “leader” end of CRISPR loci. Several CRISPR leader sequences are reported to contain a binding site for a DNA bending protein called Integration Host Factor (IHF). IHF-induced DNA bending kinks the leader of type I-E CRISPRs, recruiting an upstream sequence motif that helps dock Cas1-2 onto the first repeat of the CRISPR locus. To determine the prevalence of IHF-directed CRISPR adaptation, we analyzed 15,274 bacterial and archaeal CRISPR leaders. These experiments reveal multiple IHF binding sites and diverse upstream sequence motifs in a subset of the I-C, I-E, I-F and II-C CRISPR leaders. We identify subtype-specific motifs and show that the phase of these motifs is critical for CRISPR evolution. Collectively, this work clarifies the prevalence and mechanism(s) of IHF-dependent CRISPR evolution a
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